1. Field of the Invention
The present invention relates to an apparatus for performing circumferential seal welding between a thin tube such as a heat transfer tube in a heat-exchanger or the like and a mending sleeve inserted into that tube so as to internally cover a damaged location occurred in this thin tube.
2. Description of the Prior Art
Though it is quite natural that in steam prime motors or various electric power stations which directly made use of thermal energy, boilers as well as superheaters, feed water preheater, air preheater, condensor, etc. associated with boilers play a principal role, in various chemical industries such as the petrochemical industry and the petroleum industry, also a heat-exchanging system or a heat-exchanger is widely used in various steps of a process. Especially, a multi-tube cylindrical type heat-exchanger that is common as a large-sized heat-exchanger has a wide scope of utility, and it can well endure the use even in the high-temperature high-pressure system industry.
Now, as one of the most important associated installations in a nuclear reactor for electric power generation, a steam generator which generates steam for rotating turbine blades has been known. In general, a steam generator used in a pressurized water reactor is one kind of the multi-tube cylindrical type heat-exchangers in which secondary water is evaporated by heat-exchange with primary water heated in a nuclear reactor. A general internal structure of such steam generator is shown in FIG. 11, in which a lower end portion of a vertical type cylindrical shell 1 is partitioned by a header plate 2 to form a hemi-spherical water chamber 3, which is further divided into two chambers by means of a vertical partition wall 4, and the opposite end ports of a large number of steam thin tubes 5 extending in a U-shape penetrate the header plate 2 and are opening, respectively, in these two water chambers 3. These steam thin tubes 5 are supported within the cylindrical shell 1 by the intermediary of a plurality of support plates 6. High-temperature primary water fed from a nuclear reactor not shown enters one water chamber 3 through an inlet nozzle 7 communicating with that water chamber, then reaches the other water chamber 3 through the steam thin tubes 5, and flows back to the nuclear reactor through an outlet nozzle 8 communicating with the last-mentioned water chamber 3, and during the period when it passes through these steam thin tubes, it performs heat-exchange with secondary water for actuating a turbine which water is fed into the cylindrical shell 1 through a water feed nozzle 9. The secondary water that has become high-temperature steam in the above-described manner is adapted to be furnished to a steam turbine not shown from the top portion of the cylindrical shell 1.
Normally, in an installation relating to atomic energy, in addition to that the materials used in the installation are severely selected, quality control for the materials is carried out very strictly. In the above-described steam generator also, during its operation various inspections and repairs accompanying the inspections are conducted periodically, and in the event that anomaly of a steam thin tube should be found in one of the steam thin tubes, treatment for removing it from service as described in the following is effected. That is, for the purpose of preventing the high-pressure primary water from leaking out externally of the steam thin tube, blind plugs are fitted to the opposite end ports of the particular steam thin tube, seal welding is effected around the blind plugs, and thereby this steam thin tube is closed.
According to the method of blocking heat transfer tubes for which damage has been discovered in a multi-tube cylindrical type heat-exchanger, there is a fear that as the number of damaged heat transfer tubes increases, degradation of the capability of a heat-exchanger may become remarkable and the efficiency of the entire plant also may be deteriorated. Therefore, an alternative method was proposed, in which in place of the blind plugs a mending sleeve 102 is inserted into a heat transfer tube 103 so as to cover a damaged location 101 and the opposite end portions of this mending sleeve 102 are circumferentially seal-welded to the heat transfer tube 103 as shown in FIG. 12.
However, in the last-mentioned method, in the case where the material of the heat transfer tube is special material, it is necessary to use adequate high-temperature solder material, and so, in addition to difficulty in working, often the metallurgical structure of the heat transfer tube and the mending sleeve would be changed due to the high temperature upon welding. Hence, another method has been also proposed, in which in place of the aforementioned welding, an annular protrusion is formed around the outer circumferential surface of the mending sleeve, and air-tightness is maintained by expanding the mending sleeve to make the protrusion bite into the inner circumferential surface of the heat transfer tube. However, even according to the last-mentioned method, in the case where the heat transfer tube has been already expanded, the expanded portion is work-hardened, hence in view of the necessity of making the protrusion of the mending sleeve bite into the inner circumferential surface of the heat transfer tube, the material of the mending sleeve must be sufficiently hard as compared to the material of the heat transfer tube, and thus there exist various restrictions.